1. Field of the Invention
The present invention relates to a planar display device using liquid crystal or light emitting elements, and in particular, to an input sensor containing display device that contains input sensors.
2. Description of the Related Art
A liquid crystal display device comprises an array substrate on which signal lines, scan lines, and pixel transistors (for example, thin film transistors) are provided in a line and a driving circuit that drives the signal lines and the scan lines. Recent progress and development of integrated circuit technologies have put to practical use a process technology for forming a part of the driving circuit on the array substrate. This has served to reduce the weight, thickness, and size of the whole liquid crystal display device. As a result, the liquid crystal display device is widely utilized in various portable apparatuses such as cellular phones and notebook computers.
A liquid crystal display device has been proposed in which photoelectric conversion elements are arranged on the array substrate to provide an additional image loading function (see, for example, Jpn. Pat. Appln. KOKAI Publication Nos. 2001-292276 and 2001-339640).
In a liquid crystal display device of this kind, the amount of charge in a capacitor connected to the photoelectric conversion element varies with the quantity of light received by the photoelectric conversion element. Image loading is carried out by detecting the voltage across the capacitor.
Recently, technologies have been developed which form pixel transistors and a driving circuit on a glass substrate using a polysilicon process. The polysilicon process can also be used to easily form the photoelectric conversion element adjacent to each pixel transistor.
If display elements and photoelectric conversion elements (photosensors) are built into the liquid crystal display device, it is necessary to have a display data processing device for driving the display elements and a data processing device for processing data read from the photoelectric conversion elements. Moreover, if contents shown by an image displayed by the display elements are associated with the contents (timings, coordinate positions, and the like) of the read data (pickup image data), an associated data processing device that processes the associations is required. Further, since the contents of the read data vary depending on a user's operation, it is important that the associated data processing device have various applications.
Further, if data is to be read from the photoelectric conversion element, its read speed may create a problem. For example, when the display device is used as an operation panel, touch operations may be frequently performed at high speed. To sense such high speed touch operations, data reads must also be carried at high speed.
Furthermore, the above optical sensor is composed of a low-temperature polysilicon thin film transistor (Poly-Si-TFT). To obtain an output from the optical sensor, amplifiers are normally used because only a small current is available for optical responses. However, in the display device, since TFT amplifiers are constructed on the glass substrate, there are marked differences in characteristics (or detection capability) between amplifiers. With such marked differences in characteristics between the amplifiers, the results of detections are not reliable. This may cause the results of detections to be incorrectly recognized. Therefore, the reliability of the product may be degraded.